1. Field of the Invention
The present invention relates generally to magnetic field sensors, and more specifically, to a circuit that nulls common-mode voltage variation for a rotating terminal magnetic field sensor.
2. Background of the Invention
Hall effect sensors and other semiconductor magnetic field sensors are widely used in applications in which it is desirable to provide a measurement of DC magnetic fields and relatively low frequency AC magnetic fields that are not otherwise easily sensed with coils or other antennas. Such applications include position and motion sensors for both linear and rotational motion, power supply and motor control applications in which the transformer or motor fields are detected, audio speaker applications in which the strength of the speaker's signal-induced field is detected, and lighting controllers for high-frequency energized lamps, such as sodium lamps.
Hall effect sensors operate by providing a layer of semiconductor material with a bias current applied across one axis and sensing a voltage across the other axis. When a magnetic field is present, the uniformity of the current in the layer of material is distorted, causing non-uniform voltage distribution along the material and a differential voltage to appear across a pair of sensing terminals. To improve performance, the terminals receiving the bias current can be rotated by interchanging them with the terminals used to sense the output voltage by using a switching network, effectively “rotating” the position of the sensor terminals. The resulting output signal is modulated to a higher carrier frequency, which can be used to eliminate the effect of low frequency flicker noise in the sensing amplifier circuit and average out variations in the semiconductor material. The number of terminals can be increased from four to any number to further reduce the impact of material characteristic variations.
More recently, sensor implementations have been proposed that impose an AC magnetic field on the sensor using a current loop, which can then be detected at the sensor output and used to calibrate the sensor circuit. One or more current loops is implemented around the sensor and stimulated with a signal having known characteristics, inducing an AC magnetic field at the sensor, which causes an AC voltage of the same frequency profile in the sensor output voltage. The sensor output can then be detected and the gain of the sensor circuit adjusted to calibrate the sensor. Such circuits still use a static bias current to generate a voltage gradient that is distorted by the magnetic field to generate the sensor output voltage variation, and the bias current terminals and voltage sensing terminals can also be rotated as described above.
However, rotation of the sensor voltage causes artifacts that can disrupt the operation of the amplifier used to amplify the output voltage signal. Since the sensor material has non-uniformity, the terminal pairs have some capacitance, and since DC electric fields present at the sensor will also cause a static voltage difference between the output terminal sets that are being rotated, the effective common-mode voltage across each terminal pair can deviate substantially. When the terminal pairs are switched, the differences in common-mode voltage cause a transient at the output of the sensing amplifier, which for high rotation frequencies can become a substantial portion of the interval between rotations during which the signal is measured. Further, for low bandwidth detection applications, the settling time for the transient error to pass can be substantial. Since the transient error is multiplied by the common-mode rejection ratio of the sensing circuit for all of the frequencies of the transient, in some applications, the presence of the transient is intolerable.
Also, the rotation of the sensor voltage causes differential transients in the input circuit, as the non-uniformities in the material and external fields as described above will shift not only the common-mode voltages, but the magnitudes of the detected output voltage. Differences between the terminal capacitances, output resistance and switch resistance and the above-described static potential differences will cause differential transients in addition to any common-mode transients, when the terminals are rotated.
Therefore, it would be desirable to provide a semiconductor magnetic field sensor measurement circuit having a rotated bias and output voltage terminal connection to the sensor, in which the effects of the common-mode voltage at the sensor terminal pairs is reduced or eliminated and in which common-mode and differential transient changes in the output voltage are prevented from disrupting measurements.